The invention concerns the recovery of metals such as aluminum from secondary sources. More specifically it concerns means for the recovery of aluminum from aluminum dross which forms on the surface of molten aluminum during melting operations.
In the course of conventional aluminum melting operations, oxides, nitrides and other non-metallic impurities accumulate on the surface of the molten metal. Prior to tapping the molten metal these non-metallics are removed or skimmed from the surface of the melt. Substantial quantities of aluminum metal will be unavoidably entrained and removed with the non-metallics. This mixture of non-metallics, free aluminum and aluminum alloy is termed aluminum dross or skim.
Dross typically contains from 30 to 90 percent free molten aluminum depending on the particular processing technique and type of furnace. The balance consists of a sponge-like network of solid particles containing aluminum oxide and various other oxides, nitrates and carbides which form during the melting operation by reacting with the ambient atmosphere. This network of solid particles will be referred to as the solids network.
The recovery of aluminum from secondary sources such as dross is desirable because the production of aluminum from its primary source, bauxite, is relatively expensive due to the high energy consumption involved. However, separation of the free metal from the non-metallic portion of the dross is generally difficult and efforts to devise cost effective methods of recovery continue. For example, U.S. patents of recent years disclose activity in several phases of dross management including: improved methods of metal separation from the cold solified dross; "conditioning" of the dross during its collection and cooling; and also direct or immediate metal recovery from the hot dross. Methods claiming an improvement in total recovery or separation may be a combination of two or more of these approaches.
For example, Cromwell (U.S. Pat. No. Re. 31,028) is directed to the cleaning and preparation of cold solidified dross by more effective removal of oxide coatings on the dross. The dross is sequentially fed through various mechanical rolling and milling stages so as to separate aluminum oxide dust from aluminum concentrates. The aluminum concentrates may be utilized in conventional furnace recovery methods to produce aluminum ingot or may be further processed through selected stages to produce high quality aluminum pellets. Cromwell suggests that his method may be used with advantage for dross which has been partially prepared by known processes such as rapid cooling to reduce oxidation and thermite reaction losses.
Weiss, in U.S. Pat. No. 4,394,978, is more concerned with conditioning of the dross prior to separation and metal recovery. He cools the dross in a controlled atmosphere so as to minimize oxidation losses and to produce granules which are then ground and screened to remove dust and to improve the yield of metallic aluminum.
Roth, U.S. Pat. No. 4,386,956, in a more direct approach to metal recovery, "squeezes" or compresses hot dross using a wedge-shaped plunger or ram in a tray of matching shape. Some free aluminum drains through holes in the tray when it is filled. Compression and compaction of the dross causes additional aluminum to flow through the openings, and, during compression, the heat sink effect of the tool (wedge and tray) quickly cools the dross so as to diminish aluminum metal losses due to thermite and oxidation reactions. The compression aids in quick solidification of the dross and pooling of large regions of metal near the edges or surfaces of the dross material. Compression causes small droplets of aluminum dispersed throughout the dross to coalesce into large plates at the surface of the dross. This free aluminum metal is recovered in subsequent steps by breaking up the bulky dross pieces by passing them through a series of blasters and separators.
Van Linden, in U.S. Pat. No. 4,540,163, in part refines the method of Roth by so shaping the ram and the dross holding tray that, in operation, upward movement of the dross is inhibited, downward movement of the free aluminum is helped, and internal shear forces in the dross are maximized, so as to improve the separation and coalescing of free aluminum metal in the dross.
Although both Roth and Van Linden have the potential for greater total metal recovery by providing immediate or initial metal separation through gravity augmented by compression while the dross is in the receiving tray, they have the disadvantage of being batch operations and of producing solidified dross which is in bulky pieces requiring costly and laborious reduction before further treatment.